The present invention relates to a combustor capable of burning two fuels in compressed air. More specifically, the present invention relates to a combustor for a gas turbine capable of operating on either a gaseous or liquid fuel that significantly reduces the amount of NOx produced by combustion.
In a gas turbine, fuel is burned in compressed air, produced by a compressor, in one or more combustors. Traditionally, such combustors had a primary combustion zone in which an approximately stoichiometric mixture of fuel and air was formed and burned in a diffusion type combustion process. Additional air was introduced into the combustor downstream of the primary combustion zone. Although the overall fuel/air ratio was considerably less than stoichiometric, the fuel/air mixture was readily ignited at start-up and good flame stability was achieved over a wide range in firing temperatures due to the locally richer nature of the fuel/air mixture in the primary combustion zone.
Unfortunately, use of such approximately stoichiometric fuel/air mixtures resulted in very high temperatures in the primary combustion zone. Such high temperatures promoted the formation of oxides of nitrogen ("NOx"), considered an atmospheric pollutant. It is known that combustion at lean fuel/air ratios reduces NOx formation. Such lean burning, however, requires that the fuel be well distributed throughout the combustion air without creating any locally rich zones. Unfortunately, the geometry associated with such fuel distribution creates a complex structure that makes the incorporation of a dual fuel capability into the combustor extremely difficult.
It is therefore desirable to provide a combustor that is capable of stable combustion with very lean mixtures of fuel and air, so a to reduce the formation of NOx, and that is capable of operation on liquid as well as gaseous fuel.